1. Field of the Invention
The present invention relates generally to semiconductor devices and methods of fabricating the same, and particularly to semiconductor devices having diodes and methods of fabricating the same.
2. Description of the Background Art
In recent years, inverter devices used for example in the field of industrial electric power devices employ an insulated gate bipolar transistor (IGBT) or a similar switching element with a freewheeling diode connected thereto in parallel. One such freewheeling diode is a PIN diode. Conventionally, as such PIN diodes, there exist a diffusion diode and an epitaxial diode.
A diffusion diode has an n buffer layer formed by diffusing an n type impurity from a surface of an n type semiconductor substrate. The diffused impurity forms the n buffer layer with an impurity concentration profile having a gentle slope. Accordingly, when the PIN diode switches from an on state to an off state, or at recovery, a reverse recovery current flowing through the PIN diode in an opposite direction gradually decreases. Such recovery is referred to as soft recovery.
Furthermore, an epitaxial diode has an n− epitaxial layer formed on a surface of an n type semiconductor substrate by epitaxial growth to serve as a drift layer. In the epitaxial diode, the semiconductor substrate serving as a cathode has a high n type impurity concentration. As such, it is believed that a modulation level causing conductivity modulation increases and forward voltage can be decreased. A document disclosing a diffusion diode is for example Japanese Patent Laying-open No. 2007-059801.
However, conventional semiconductor devices have disadvantages, as follows: First, a diffusion diode has an n buffer layer having an impurity concentration profile gently decreasing as seen from a cathode toward an anode. This provides a nature that hardly increases the modulation level and provides an increased on resistance, and cannot decrease forward voltage. Furthermore, the n buffer layer is relatively large in thickness, and this provides a nature providing an increased total amount of carriers that are discharged at recovery, resulting in increased switching loss (or recovery loss).
Furthermore, an epitaxial diode has an impurity concentration profile rising steeply from an n− epitaxial layer to an n type semiconductor substrate, and this provides a nature decreasing electrons that have been injected in the on state at recovery rapidly so that oscillation is facilitated.
On the other hand, semiconductor devices applied to some inverter circuits are required to reduce forward voltage, rather than recovery loss, or vice versa.